removes channels_builtin when enabling nimPreviewSlimSystem (#20713)

lib/impure/rdstdin.nim

@@ -22,6 +22,9 @@ runnableExamples("-r:off"):
     if line.len > 0: echo line
   echo "exiting"
 
+when defined(nimPreviewSlimSystem):
+  import std/syncio
+
 when defined(windows):
   proc readLineFromStdin*(prompt: string): string {.
                           tags: [ReadIOEffect, WriteIOEffect].} =

lib/pure/collections/deques.nim

@@ -47,7 +47,6 @@ runnableExamples:
 ## See also
 ## ========
 ## * `lists module <lists.html>`_ for singly and doubly linked lists and rings
-## * `channels module <channels_builtin.html>`_ for inter-thread communication
 
 import std/private/since
 

lib/pure/concurrency/threadpool.nim

@@ -14,7 +14,6 @@
 ## See also
 ## ========
 ## * `threads module <threads.html>`_ for basic thread support
-## * `channels module <channels_builtin.html>`_ for message passing support
 ## * `locks module <locks.html>`_ for locks and condition variables
 ## * `asyncdispatch module <asyncdispatch.html>`_ for asynchronous IO
 

lib/system.nim

@@ -2206,7 +2206,8 @@ when notJSnotNims and hasAlloc:
     include "system/repr"
 
 when notJSnotNims and hasThreadSupport and hostOS != "standalone":
-  include "system/channels_builtin"
+  when not defined(nimPreviewSlimSystem):
+    include "system/channels_builtin"
 
 
 when notJSnotNims and hostOS != "standalone":